Modern motor vehicles often have a tank venting system. By this, the fuel vapors which arise in a fuel tank are adsorbed in an activated carbon container. To regenerate the activated carbon container, a tank venting valve, which is located in a connecting line between the activated carbon container and an induction pipe on the internal combustion engine, is opened during operation of the internal combustion engine so that the fuel vapors stored in the activated carbon container are introduced, by a flushing effect, into the intake ducting of the internal combustion engine and take part in the combustion. This results in a change in the composition of the combustion mixture.
For the purpose of avoiding a raised discharge of pollutants due to this change in the combustion mixture, the known approach is to correct correspondingly the quantity of fuel which is dosed into the combustion chambers of the internal combustion engine by means of injection valves. This corrective operation is also known as an injection quantity correction. However, it is only possible to carry out the injection quantity correction if the amount of fuel fed in by the fuel vapors is known. In accordance with one known method, the level of charging of the activated carbon container with fuel vapor is determined for this purpose. This is effected by slowly opening the tank venting valve over an opening ramp with a prescribed timing. The charge in the activated carbon container and the amount of fuel fed in by the fuel vapors are determined on the basis of the deviation between an output value from a lambda regulator in the motor vehicle after the tank venting valve is opened and a reference value which corresponds to the exhaust gas composition before the tank venting valve is opened.
Determining the charge level of the activated carbon container whilst at the same time avoiding a larger discharge of pollutants calls for a very slow opening of the venting valve, and is therefore time-consuming. In order to permit a larger number of regeneration phases for the activated carbon container, there are known methods by which the tank venting valve is opened at different speeds depending on the length of time which has elapsed since the last regeneration phase. If the time which has elapsed since the last regeneration phase is very short, the assumption is made that the level of charge of the activated carbon container calculated at the last regeneration phase, and the reference value on which the calculation of the charge level is based, are still valid. This permits a very rapid opening of the tank venting valve whilst carrying out an injection quantity correction based on the charge level and reference value which are already known. In the case of a regeneration phase which lies further back, a new determination of the charge level of the activated carbon container is necessary, and both the reference value and the charge level must be predetermined. This in turn calls for a very slow opening of the tank venting valve. The frequency of regeneration phases for the activated carbon container can be raised by the method cited above. However, the rigid differentiation between two speed levels for the opening of the tank venting valve, and the determination of the reference value as a function of the length of time which has passed since the last regeneration phase can prove to be inflexible and, in respect of reducing the discharge of pollutants, inadequate.